Field of the Invention
The invention relates to a device for collecting and cooling a reactor melt or reactor-meltdown products from a reactor pressure vessel, having an expansion chamber in which the spreading reactor-meltdown products can be cooled with the aid of a coolant, such as cooling water.
The device is usable particularly in the EPR pressurized water reactor.
In nuclear reactors, safety considerations so far have assumed that failure of the reactor pressure vessel need not be feared, because of the choice of material and the dimensions. Recently, however, in the course of more-intensive safety investigations of nuclear energy utilization, the situation in which a reactor pressure vessel "fails", however unlikely that may be, has also been studied. In particular a new reactor type, the European pressurized water reactor EPR, has been based on such considerations (see the publication entitled: VGB Kraftwerkstechnik 73 (1993), No. 2, pages 97-101). In contrast to the safety philosophy followed heretofore, in that reactor type a nuclear meltdown accident, a so-called MCA (Maximum Credible Accident), is not flatly excluded as a possibility. Investigations have also been made as to whether or not steam explosions might occur during a core meltdown, and whether or not the water vapor suddenly produced in such a critical phase might not cause the pressure vessel to collapse. There is no question that such accidents, however theoretical they might be, must be made controllable.
In a hypothetical serious accident in a nuclear power plant with a water-cooled reactor, it is accordingly assumed that the reactor core will melt. Subsequently, reactor-meltdown products would escape from the bottom of the reactor pressure vessel into the shielding pit of the reactor containment. In order to enable controlling such an accident, suitable structural provisions must be made to prevent the meltdown products, possibly escaping at excess pressure from the reactor pressure vessel and collecting in the region of the bottom of the nuclear reactor plant, from causing failure of the containment.
A nuclear reactor plant that is known from German Patent DE-PS 28 40 086, corresponding to UK Patent Application GB 2 030 347 A, for instance, has a collecting device for a reactor core that is melting down. A vertical drain channel is provided below the shielding pit surrounding the reactor pressure vessel. That drain channel passes through the containment and leads into a meltdown pit disposed below the containment. There the reactor-meltdown products emerging from the reactor pressure vessel are distributed over an absorber bed formed by a steel tub filled with water-free materials. After the steel tub melts, the meltdown products reach the bottom of the meltdown pit. The bottom and the side walls of the meltdown pit are water-cooled, and thus the meltdown products gradually solidify.
In the nuclear reactor plant known from German Patent DE-PS 29 25 680, a collecting tub for receiving the meltdown products is also provided and is located below the level of the reactor foundation. The collecting tub is not located directly below the reactor core but rather is located next to it and is connected to the bottom of the reactor building through a chute that ends horizontally above the collecting tub.
A collecting device for reactor-meltdown products is known from FIG. 1 of a publication entitled: Emerging Nuclear Energy Systems 1989, Icenes 89, Karlsruhe, July 3-6, Proceedings of the Fifth International Conference on Emerging Nuclear Systems, pp. 19-24. In that device, a cooled collecting tub is disposed inside the containment directly beneath the reactor pressure vessel, and in that tub the meltdown products spread out over a large area and can cool down in direct contact with water.
Published European Patent Application 0 392 604 A1 discloses a collecting and cooling device in which a water inlet is located beneath the reactor pressure vessel. In the case of a meltdown, the reactor-meltdown products and portions of the reactor pressure vessel and its built-in fixtures would drop directly into the water bath. From the standpoint of effective cooling and the aversion of steam explosions, that is not expedient. What is sought is instead a cooling process in which the outflowing reactor-meltdown products will not meet a relatively large quantity of water.
German Published, Non-Prosecuted Patent Application DE 43 22 107 A1, the priority of which is claimed in the instant application, discloses a device of the type referred to at the outset, and especially a core retention device operating on the expansion principle, in which in the event of failure of the reactor pressure vessel the forcefulness of steam production caused by contact between the reactor-meltdown products and any water that is present is considerably reduced or even averted entirely. That device is equipped with:
a) an antechamber disposed below the reactor pressure vessel,
b) an expansion chamber for the reactor-meltdown products,
c) a channel, between the antechamber and the expansion chamber, that is provided with a partition which can be destroyed by the reactor-meltdown products, and
d) a coolant reservoir, which is connected to the expansion chamber through a closure element that can be destroyed by the reactor-meltdown products.
The expansion chamber in that case is dry under operating conditions.
Such a partition between the antechamber and the expansion chamber is also already shown in FIG. 4 of Published European Patent Application 0 563 739 A1 and B1, corresponding to U.S. Pat. No. 5,402,456. The partition includes a thin steel plate and it partitions off the outlet of the antechamber to a drain channel in the direction of the expansion chamber until it is destroyed by the heat of the meltdown products. The expansion chamber in that case is especially already filled with coolant when the reactor-meltdown products enter.
The device referred to at the outset relates to that European Patent Application, although it remains an open question whether the expansion chamber is dry or filled with water in the event of use. Currently, the EPR concept assumes that the expansion chamber is dry under operating conditions.
The invention is based on the concept that particular attention should be paid in construction to the floor of the expansion chamber, so that the reactor-meltdown products can be reliably caught in the expansion chamber and cooled there for a long time. A structure can be imagined in which the floor includes a protective or fireproof concrete, having a surface over which the reactor-meltdown products can spread, and in which beneath it a concrete base plate extending over the inner cross section of the containment is disposed. The usual containment liner, which also lines the walls of the containment, can then be located below this concrete base plate. A thick floor plate which again is formed of concrete in turn can be disposed below the concrete liner. Further investigations have shown that even with that kind of multilayered floor, special precautions against overheating must be made.
Initially, it must be remembered that in the case of failure where reactor-meltdown products occur, the containment liner that is intended to prevent the exporting of radioactivity to the atmosphere must not be allowed to become too hot. Another important factor is that the aforementioned protective or fireproof concrete and the concrete base plate should not assume an overly high temperature when reactor-meltdown products strike it, even after a relatively long time when the reactor-meltdown products are in the expansion chamber. Otherwise, as a consequence of decomposition, for instance of water bound chemically in the concrete, gases could form and escape, which under some circumstances would cause the protective concrete layer in the expansion chamber to be forced upward or lift off.